Many existing mobile radio communications systems today use a core network comprising a IP Multimedia Subsystem (IMS), or other Session Initiated Protocol (SIP) based systems containing the functionality of an IMS system, for offering packet switched services to e.g. users of mobile terminals, IP-based communication terminals or the like.
An example of such a packet switched service is the Push to talk Over Cellular (PoC) communication service that provides wireless or wireline PoC Users with a virtually instantaneous method of connecting to other PoC Users by just the push of a button on their mobile terminals. The PoC communication service is half-duplex and can be compared to the communication performed when using conventional walkie-talkies. The PoC communication service can be used for person-to-person calls as well as for group communications over cellular networks, such as GSM, WCDMA or CDMA2000 networks, and/or fixed networks, such as any other IP network or interwork with legacy Push To Talk networks.
The PoC communication service has been developed by the Open Mobile Alliance (OMA) in the OMA PoC WG, which is currently described in PoC 1.0 Release and PoC 2.0 Release.
PoC Users uses PoC Clients, comprised in their mobile terminals, to access the PoC Service. In order to communicate using the PoC Service, the PoC Client establishes a PoC Session. The PoC Session is routed through at least one PoC Server. The at least one PoC Server may perform a Participating PoC Function and/or a Controlling PoC Function. However, both are necessary in order to establish a PoC Session. As a PoC Session has been established through one or several PoC Servers, the PoC Users participating in the PoC Session may start communicating over the half-duplex PoC communication service. It should be noted that the term PoC Session may hereinafter be used to refer to a 1-1 PoC Session, an Ad-hoc PoC Session, Pre-arranged PoC Group Session or a Chat PoC Group Session or the like.
A PoC User can also participate in one or more PoC Sessions at the same time. In order to make it possible for the PoC User to only receive Media from one PoC Session at a time, a function called Simultaneous PoC Sessions has been developed as part of the OMA PoC Specifications. The OMA PoC Specifications defines a priority order, which informs the PoC Server how to prioritize the one or more PoC Sessions if Media is received for more than one PoC Session. The PoC Server may then select from which PoC Session it shall forward Media to the PoC Client in the mobile terminal of the PoC User. The priority order of the PoC Sessions may be selected by the PoC User using the PoC Client.
However, as will be described more in detail in the description of the present invention, this may lead to instances where PoC Users may be unavailable and not able to listen or take part in a PoC Session. This may cause unnecessary signalling and put additional loads on the network, since Media may be transmitted to PoC Users that are not able to listen to the Media anyway.
Furthermore, other PoC Users participating in a PoC Session may also not know if the information sent in the PoC Session has been received by its intended recipients, unless the recipient acknowledges by answering.